The first part of this chapter contains a general introduction to bacterial population genetics, and the second part reviews the available data on the genetic diversity within Helicobacter pylori and the mechanisms that have generated this diversity. Three types of population structures includes clonal, panmictic, and epidemic. Genetic diversity arises by intragenomic genetic events including point mutations, deletions, insertions, and rearrangements. As early as 1986, it was recognized that H. pylori exhibits an unusual degree of genetic heterogeneity. Compatibility matrices yielded results that were consistent with frequent recombination in H. pylori. The frequency of both recombination and purification remains to be quantitated. Biogeographical diversity of three virulence-associated genes such as cagA, vacA, and hspA could arise by various mechanisms, ranging from founder effects associated with vertical transmission to selection for antigenic or functional variants with differential fitness for certain host populations. Subsequent analyses of the same housekeeping genes from additional isolates from Indonesia and South Africa have confirmed the original conclusions, but further analyses are needed to test whether virulence genes such as cagA also fall into three populations. An initial study failed to detect genomic changes in paired H. pylori isolates obtained 2 years apart from 20 patients, but two subsequent studies have shown that H. pylori strains can indeed change during chronic colonization.

Comparison of the mean homoplasy ratio (H) and mean genetic distance at synonymous sites (DS) among different bacterial species. The sources of the data for these analyses are given in reference 1. Modified from reference 1 with permission.

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Figure 1

Comparison of the mean homoplasy ratio (H) and mean genetic distance at synonymous sites (DS) among different bacterial species. The sources of the data for these analyses are given in reference 1. Modified from reference 1 with permission.